Minimally invasive surgery (MIS) is one of the main evolutions of surgical techniques and it provides great benefits to the patient. For example, this technique reduces patient scars, infection risk, and post-operative morbidity. MIS uses endoscopic camera and instruments manipulated through small incision on the skin to avoid open surgery. However, MIS procedures are more difficult to perform than standard surgery. For example, the field of view of a traditional camera is quite limited compared to human perception. If changed to a wide-angle lens, it introduces image distortion. That is, straight lines become curved in the distorted image. This seriously affects the determination on coordinates of targets and degrades the capability of size and shape determination at the time the surgeon uses the endoscopic camera to perform MIS.
However, it is necessary to use the wide-angle endoscopic camera in MIS since a large field of view is needed such that the surgeon can be aware of each organ status and relative positions of the instruments manipulated during the surgical procedure. Therefore, a way to solve this problem is to convert the distorted images captured by the wide-angle lens into normal images (i.e., undistorted images). In addition, the reconstruction algorithms used in a 3D image reconstruction system are based on the undistorted images. One advantage to converting into the undistorted images is that it is easier to perform 3D reconstruction. The undistorted images are also needed to achieve virtual guidance surgery.
Although there are existing calibration methods for converting the distorted images captured by the wide-angle lens into the normal images (i.e., the undistorted images), there are several problems to apply them to the practical situation. In practice, current endoscope provides 60 Hz frame rate and high definition (HD) resolution. It is almost impossible to achieve real-time image correction by using only one central processing unit (CPU) for the computation. Furthermore, traditional methods are still not feasible for a zoomable lens even though the lens can be calibrated before operation or the surgery. It is because the distortion parameters used for correcting the distorted images will be changed as the focal length of lens is changed.
In conventional skills, for a lens without zooming or changing its focal length, the distortion parameters of the endoscope can be estimated offline but known methods for this need some user intervention. They are manual or semi-automatic, rather than fully automatic. Further, traditional zoom calibration methods have to calibrate the camera for all possible zoom settings so as to obtain corresponding distortion parameters, or need special hardware to get current focal length and radial distortion. These methods are not suitable for calibrating a zoomable endoscopy.